An extruded plastic product, such as a pipe, can be formed in an extrusion apparatus, by forcing molten polymer or plastic under pressure through a die to form the extruded product. The rotating screw of the extrusion apparatus can form pressure waves in the molten polymer, which can remain present while passing through the dies. Such pressure waves can leave distinct visual screw marks or cause defects on or in the extruded product, which are typically undesirable.